The present invention relates generally to optical communications, and more particularly, to a method for hitless protection in high optical transport network OTN framer and front end.
The Ensuring no traffic loss becomes more and more important in today's telecommunication systems. Internet trading is one of the applications that require packet loss to be as low as possible; real-time Internet applications such as voice over IP and video conferences are coming to people's daily life, and packet loss is undesirable for the causing of flickering noises on the phone lines or viewing distorted video clips. Service continuity is more critical comparing to traffic loss, so redundancy protection are always built into the telecommunications systems to avoid service interruption during system or link failure. Redundancy protection is the technique to provide a backup line card or fabric card and replace the primary one when failure occurs. Shown in FIG. 1 is a block diagram of an exemplary communication system 100 with line card protection, where primary line card 102 and backup line card 104 are connected to the same physical link 110 through switch 106, which is further controlled by protection control signal 108. In normal mode, switch 106 selects the output from primary line card 102, while in case failure happens in 102, the backup line card 104 will be connected to the output link 110. For the above application reasons, it is important to minimize traffic loss when switching from primary to the backup one.
Hitless protection describes the ability of switching to protecting mode without losing frame and framing synchronization when failure occurs, to ensure that telecommunications equipment provide uninterrupted or continuous service and maintain an extremely high-reliability rating. This requires bit-level aligned between primary and backup interfaces, to keep receiver side synchronized when switching from primary to backup one. Shown in FIG. 2 is a block diagram of an exemplary configuration in system 100 to support bit alignment, where tunable delay lines 112 and alignment detection circuit 114 is added for bit alignment purpose. Tunable delay lines 112 can also be integrated inside the line cards, either before or after the optical modulator. Alignment detection function 114 can either be integrated with the system, or through external method. In low-speed case, this is easier to achieve because the symbol period is wide enough to allow relatively large skew between primary and backup interface. However in high-speed case, for example 100 Gb/s line rate with QPSK modulation and polarization mode multiplexing, the serial data rate is around 25 Gb/s, which is equivalent to 40 picoseconds per symbol. It is very difficult or time consuming to align the output signals for such short bit period.
In an optical system, the switching from primary interface to the backup one may be achieved by shutting down the light from primary line card and enabling the output from backup line card, or using an optical switch. Overlapping of two optical signals may cause corrupted signals in receiver side due to beating noise, and the switching from one to another may encounter certain delay that causes signal loss. So in any case the receiver side may experience signal loss during the switching time. This signal loss will cause receiver side loss synchronization, and it will take long time to get re-synchronized.
To describe the actual effect for the above mentioned problem, here we give an example using an optical transport network (OTN). OTN uses frame alignment signal (FAS) for frame aligning. The OTN frame is of fixed size (say size L), and FAS field is located at the beginning of each frame. In an example OTN receiver, after system reset, it searches for FAS within the incoming data stream. After first FAS match, the internal data and phase alignment is adjusted to the newly found FAS. The receiver keeps on checking the FAS pattern for certain period, and in case all the checking matches, it will reach in-frame state. In this state it keeps on monitoring the FAS pattern for each frame, and if there is mismatch for a pre-configured number of frames, it will switch into out-of-frame state which means loss of frame synchronization. When switching from primary line card to the backup one, bit misalignment will cause bit(s) missing or duplication. Either case will cause continuous FAS checking failure and finally enter out-of-frame state which makes the system fail to support hitless protection. Signal loss will lead to the same result as bit misalignment.
Accordingly, there is a need for provides a solution to eliminate the constraint of exact bit and phase alignment, and to quickly recover in case of signal loss, by modifying the operating procedure in receiver side.